Phthalic anydride is manufactured industrially by gaseous oxidation of o-xylene by passing the o-xylene/air mixture through a tube reactor, where the tubes are filled with catalyst that contain vanadiumpentoxide and titanium oxide. In the partial oxidation of orthoxylene or naphtalene to phthalic anhydride it is very important to obtain highest possible hydrocarbon load, with still good quality coming out from the reactors.
In order to optimize the catalyst, it is very important to control the reaction in the way to achieve highest possible hydrocarbon load (g/Nm.sup.3) to be converted into best possible quality at highest possible yield. Depending on the type of catalyst (activity and selectivity) the possibilities will vary rather wide with existing catalysts on the market. All existing catalysts are based on vanadium pentoxide and different moderators are involved to make the different catalysts to operate differently concerning sensitivity, activity and selectivity.
To control the reaction and the activity of the catalyst, it is very important to keep the saltbath at the right temperature. The bath is flowing around the tubes in order to remove the heat created from the reaction inside the tubes, which are filled with catalyst. Normally the saltbath temperature will be in the range of 370-400.degree. C. over the life-time of the catalyst. To be able to produce a good quality the reaction inside the tubes must be held at a rather high temperature level (450-470.degree. C.) which means that a lot of byproducts will be created (specifically oxidation products like CO, CO.sub.2 and maleic anhydride) which are the biggest impurities in this reaction by weight. That fact will reduce the yield from hydrocarbon to phthalic anhydride and furthermore at these high temperature conditions it would be very difficult to hold the reaction "steady" and to optimize the reaction conditions.
During recent years the development has moved towards higher hydrocarbon load (g/Nm.sup.3) while the reactors today look very much the same as in the past. The producers of catalysts have tried to compensate these higher loads by changing the recipes.
New catalysts have been introduced which are splitted in the activity in the way that reaction (temperature profile) will be more spread out over the reaction tubes, but still the reaction will be very sensitive to variations and of course very much depending on the quality of the catalyst (activity, selectivity). This means that today it can be very difficult to operate the reactors with high load and good quality because the sensitivity of the oxidation has increased.
Another way to overcome the difficulties is the use of two or more reactors instead of one reactor. Such solution is disclosed in EP-patent application 0686633. According to this application two reactors are applied and the gas composition at the inlet of the first reactor is controlled in the range of the lower inflammability limit, while the gas composition at the inlet of the second reactor is controlled closer to the range of the upper limit of inflammability. Both reactors and more specifically the second reactor are preferably tubular reactors cooled by molten salt circulation. The tubes of the second reactor are substantially longer than the tubes of the first reactor. The reactors are applied so that feedstock is added also to the effluent of the first reactor and the gas mixture is then introduced into the second reactor. Providing two reactors which are both salt-cooled leads to high investment and operating costs and besides, in existing plants available space is often limited.
A reference is made also to European patent application 0453951, where the reactor is divided two or more reaction zones following each other and by using same or different catalyst in each zone. Also in Austrian patent application 9201926 a second reaction zone is disclosed. In this case the effect obtained by said construction is insufficient and requires specially manufactured honeycomb catalyst structures.